Electrospray ionization (ESI) is the predominant technique for vaporizing and ionizing a liquid-phase sample containing molecular species to be input into a mass spectrometer for mass measurements. It has the advantage of being a “soft” ionization technique which creates ions of the intact molecules with minimal molecular fragmentations. In electrospray ionization, the molecules carried in a liquid buffer are pumped through a small tube or capillary toward the inlet region of a mass spectrometer. The opening of the tube, which may be of a variety of designs, called the spray emitter, is placed in close proximity, from under a millimeter to a few centimeters, of the mass spectrometer inlet. The liquid buffer and the molecules it carries vaporize and ionize under a sufficiently large electric field created by a voltage difference from about one to about five kilovolts between the spray emitter and the mass spectrometer inlet. The charged ions in the sprayed eluates, materials that elute from the spray emitter, are directed into the mass spectrometer inlet so that the chemical species in the eluates can be identified according to the ratios of the masses of the chemical species to the electric charges they carry.
To assist the desolvation of the ions before the ions enter the mass spectrometer, a high pressure gas called a sheath gas typically of research grade nitrogen or purified air is sometimes supplied axially and concentrically with the liquid flow so that the gas streams may strip away the water droplets or solvent molecules from the gas-phased solvated ions of interest resulting in enhanced sensitivity of the mass detection measurement. A high temperature created by heaters and other gases not axially directed as the liquid flow may also be applied to aid desolvation of the charged ions. If the molecules of interest are in a mixture, then high performance liquid chromatography (HPLC) is the most widespread technique used to separate the molecules in the mixture carried in a flowing liquid mobile phase based on their interactions with the stationary phase which is typically particles or other materials packed into a tube or capillary called a column. The spray emitter is connected to the exit end of the chromatographic column so that the eluates from the column can be vaporized and ionized with ESI for mass spectrometry analyses.
Mass spectrometry analyses of molecular species have advanced rapidly such that the detection limit of mass spectrometers can reach attomole (10−18) level quite routinely when the flow rate of the liquid buffer is in the sub-microliter per minute or nanoflow regime. In this flow rate regime, the eluates can be vaporized and ionized under the applied electric field alone, i.e., no desolvation of ions using sheath gas or any other gases, or high temperature is needed. The resulting spray at this flow rate range is called nanospray, and the spray is characterized as a cone-jet mode since the spray appears to be drawn to a point slightly away from the spray emitter opening and burst into a cone-shape jet of fine mist carrying ions not requiring extensive desolvation before mass spectrometry detection. A popular spray emitter design in the art is made by melting the front end of a fused silica capillary with thermal energy either from a laser or a flame torch while pulling axially with a force so that the melted fused silica elongates and breaks, forming a taper with a small opening. For example, a fused silica capillary with an inside diameter of 75 μm and an outside diameter of 360 μm may be pulled to a taper over a few mm to about 1 cm to form an opening at the end that is a few μm to under 20 μm in diameter with an outside diameter that may be from about 10 μm to about 100 μm. The capillary which has a uniform inside diameter before pulling now has a gradually narrowing inside diameter in the tapered region. The taper opening generally creates a good spray, but the taper opening may change shape or may be damaged when used for nanospray operation over a period of time ranging from a few minutes to a few weeks. The spray tips with an opening of smaller than 10 μm in diameter may be prone to clogging and are structural fragile. Furthermore, the elongated channel of diminishing diameters inside the taper is also conducive to clogging. Spray emitters in the sub-microliter/minute flow-rate range of other designs are not as widely used than the melt-pulled fused silica emitter because the spray quality is considered not as good or consistent. One such spray emitter design is made of a stainless-steel tubing or a fused silica capillary tapered only on the outside wall leaving the inside diameter of the tubing uniform in size. Still another spray emitter in the sub-microliter/minute flow rate regime is a plastic injection-molded nozzle with a conical channel for conducting the liquid buffer to the spray opening. Still another design is just a flat-cut thin-walled fused silica capillary 20-25 um in inside diameter and 90-100 um in outside diameter. This flat-cut fused silica capillary is typically used with sheath gas to assist spray even in the sub-microliter flow rate range because it does not produce the cone-jet mode of spray. This flat-cut fused silica emitter is extremely fragile to handle due to its small diameters, and the sensitivity of detection from this spray emitter is not nearly as good as those capable of cone-jet mode spray. In addition, all the spray emitters in the art, even if they are capable of cone-jet mode spray for one range of liquid buffer compositions, usually from pure aqueous to pure organic solvent with some minor additive ingredients, and at one polarity of the high spray voltage, are not necessarily good at a different liquid buffer composition or high voltage polarity. In particular, negative ion spray is considered challenging for all the nanospray emitters in the art.
For the higher flow rate regimes, e.g., from a few microliters/minute to more than one milliliter/minute, the spray emitters in the art are made of a blunt-ended. i.e., untapered, stainless steel tubing, and the vaporization and ionization are assisted by high pressure gas and high temperature heaters around the spray region and also along the liquid and gas flow paths directly behind the spray emitter opening. These spray emitters are not capable of the cone-jet mode spray and as a result, the ionization efficiencies of these flat-ended stainless-steel spray emitters are considered less than that of the spray emitters producing cone-jet mode sprays in the nanospray regime. The sensitivity of the mass spectrometer detection of the molecular species in the higher flow rate regimes would have been compromised if not for the fact that the concentration of the molecules of interest can be increased substantially in most high flow-rate applications since sample availability is typically not a limiting factor, contrary to applications in the nanospray regime which are focused on proteins and peptides. Because of environmental concerns, there is a strong desire especially in the pharmaceutical and biotech industries to reduce the use of large amounts of solvents in liquid chromatography-mass spectrometry analyses, especially those exceeding 1 mL/minute. At the same time there is a push to attain the high sensitivity detection that has been achievable only with nanospray mass spectrometry because of the more stringent requirements in applications such as toxicity screening. However, nanoliters/minute flow rates for mass spectrometry sample input are considered both not robust enough and also too slow from the standpoint of high throughput analysis because the fused silica spray emitters are too fragile, and the stainless steel and plastic spray emitters have not been established to spray reliably and consistently so that human intervention during analyses is required.